Resolution Limits?

This has to do with our tendency to prefer round numbers. The
"resolution" is very often determined by 2theta(max). For 2theta(max=60
deg, the calculated resolution will be the wavelength.
Resolution is not a sharply defined quantity, so you are right: there is
some measure of "arbitrariness." Normally it is well-reasoned
arbitrariness of no great scientific consequence.
Hakon Hope